I personally would refrain from using railroad ties. Because of the chemicals and oils they have. Sure people can make cases one way or the other on chemicals and their leeching. However, I cannot justify it in my own mind.
Here is a great article on root depth:
http://www.soilandhealth.org/01aglib...010137toc.html
Many plants have taproots and that will cause them to go deeper than 6". I am not sure how 6" became the standard, but it seems to work most of the time.